Sorry if this has already been posted–I can’t remember, atm, and I can’t find it in a search–but is it possible to have individual custom dictionaries?
I ask because what I write tends to have a lot of coined/abnormal words, and what’s proper spelling for one project isn’t always proper for another.
Thanks!
Not really. The system stores your custom spelling words in a file in your library. You can edit this with a text editor that displays NULL characters, and I believe there is also a free tool that lets you play with the file manually. However there is no system for having multiple dictionaries or dictionaries for specific applications on the system. The whole OS uses the same dictionary. The best you can do is “fork” the file. Duplicate it before a project, and leave that copy sitting there. When you are done with the book, delete the one with all of the names and such in it and go back to the copy by changing its name to remove the “copy” from it. Of course if you do this, you’ll lose any genuine word updates that were made during the writing of the book as well.
It would probably just be easiest to use one of the mentioned tools above to remove the words you don’t want from the file, later on.
The file is located in Library/Spelling. If you are multi-lingual, you’ll likely see several entries, one for each language.
Thank you, Amber! 
My current WiP is going to be a pain come spellcheck time, regardless how I work with it, thanks to the fantasy terms and smatterings of foreign languages in it. (Serves me right for having characters who speak Italian, Finnish, Spanish, and/or Japanese.)
starts thinking…
EDIT: Would MMD have something comparable CSS’s div classes?
Could you elaborate a bit on that last question? I’m not sure how CSS relates to the system spell checker. 
Sorry. I was thinking that I could potentially define each of the segments of each language as a “class”—like surrounding any section with Italian as or something like that, in a way that could possibly be used later to separate out the different text languages from each other.
Or I suppose I could use HTML spans (or an equivalent) to define lang. Hm… 
It seems to me like that should be doable; I’m not sure how to pull it off, though.